linear programming

I have a part of a problem which states that:
the exposure rating for an ad is 90 and new customers per ad is 4000 for the first 10 ads shown and then it declines to 55 and 1500 respectively after the initial 10.

i have used 2 variables for the differences in exposure ( t1 for number of ads that are 10 and under and t2 for everything after 10) my question is how do i actually write that part of the constraint out?